Thus the rationale behind alternating strength and cardio work?
I never used a rationale on it, Doc. I just figure when you get tired of doing one thing, you start doing something else.
As long as you keep standing up, you're winning.
Mostly. Depends on what goals you have set for yourself. Me? I just wanna look good. I don't care what my weight is as long as I don't look like a slob. If I hit 300, I'm fine with that as long as it also looks like I could exit a building without having to worry about a door being present.
You can either train a muscle for size, or you can train it for stamina. Doing both at the same time isn't optimal for either. Instead, alternating your workout plans helps train up each area individually and optimally. I normally only do enough cardio to count as a warm up. About 15 minutes at a medium-high intensity. Any other stamina I garner relates only to how many of what type of repetitions I do.
Training for stamina uses almost no weights at all other than body weight. Running. Biking. Spinning. Ect.. all concentrate more on the heart and lungs than it does on strength. Being able to continue to get those muscles to twitch is a completely different beast than trying to get a maximal contraction out of that same muscle.
Some try and do both at the same time. To the best of my knowledge, this can garner some seriously mixed results. You will either be too pumped up to keep up a cardio workout, or too exhausted to lift effectively. Best to just split up your training program over a course of month to alternate back and forth every few weeks. I would recommend a program that alternates every two months. This is a little longer than what some might recommend, but I don't burn out quite as fast as some do either.
My 2 cents. Worth the electrons its printed on... ;-)